Field of the Invention
The present invention relates to a voltage regulator. The output voltage of which depends on the drive to a transistor contained in the voltage regulator.
A voltage regulator of this type is shown in FIG. 5.
The configuration shown in FIG. 5 contains a direct voltage regulator and a load impedance Zout connected thereto.
The voltage regulator contains a differential amplifier (a differential transconductance amplifier) OTA1, an NMOS transistor MN1, a first resistor Rfb, a second resistor Re, a third resistor Rs1, a first capacitor Cs1, a second capacitor Cs2, and a third capacitor Cs3.
The voltage regulator generates an output voltage Vout which is picked up at a source terminal of the transistor MN1 and which is supplied as a supply voltage to the load Zout. A supply voltage supplying the voltage regulator with power is applied to a drain terminal of the transistor MN1, and the gate terminal is connected to the output terminal of the transconductance amplifier OTA1. The transconductance amplifier OTA1 has two input terminals, one of which is supplied with an input voltage Vin and the other of which is supplied with a voltage depending on (fed back from) the output voltage Vout. The transconductance amplifier OTA1 forms the difference between the voltages and outputs the result to the gate terminal of the transistor MN1. The voltage fed back is picked up at a node x2 located between the resistors Rfb and Re. The resistors Rfb and Re are connected in series and are disposed between the source terminal of the transistor MN1 and ground.
FIG. 6 shows the small-signal equivalent circuit of the configuration shown in FIG. 5.
The voltage regulator described is a series voltage regulator with a common-drain NMOS transistor as a driver stage. It should be clear, and does not require further explanation, that the voltage regulator shown is capable of generating a constant output voltage Vout that depends only on Vin and the feedback factor (determined by the resistors Rfb and Re). However, this is not guaranteed under all circumstances, especially in the case of complex loads Zout, i.e. in the case of loads with inductive and/or capacitive components. The system may become unstable in this case.
The stability problems would not occur if it could be ensured, by suitable dimensioning of Rfb and Re, that the current Is1 flowing through the transistor MN1 does not drop below a certain minimum value even with a large Zout, that is to say a low load current, that is to say the transistor MN1 has a certain minimum transconductance (a certain minimum output conductance). However, providing a large (shunt) current flowing via the transistor MN1 and the resistors Rfb and Re is associated with various disadvantages. In particular, such a voltage regulator has a high intrinsic power requirement, and the transistor MN1 has to be configured to be larger than would be the case with a low shunt current. In addition, the minimum shunt current necessary for ensuring the stability is not available for driving the load Zout.
The dependence of the stability of the voltage regulator on the minimum shunt current is now explained.
In a simplified way, the configuration according to FIG. 5 can be understood to be a two-pole system. The stability criterion requires that the two poles are apart by a factor of at least nxe2x89xa710.
The first pole fp1 is obtained in a simplified manner in accordance with equation 1.1.                               f          p1                ≅                  1                      2            *            π            *                          C              ml                        *                          1              /                              gm                OTA1                                                                        (        1.1        )            
It can be seen that the first dominant pole is determined by the transconductance gm of the transconductance amplifier OTA1 and by the stabilization capacitance Cm1. In practice, the first pole is invariant and is determined by the necessary bandwidth of the configuration.
The second pole is determined in a simplified manner by the load capacitance Cout at the output Vout, the load impedance Zout and the output conductance gds of the driving transistor MN1. Equation 1.2 reproduces the mathematical relationship for calculating the second pole.                               f          p2                ≅                  1                      2            *            π            *                          C              out                        *                          (                                                1                  /                                      gds                    MN1                                                  ⁢                                  "LeftDoubleBracketingBar"                  Zout                  "RightDoubleBracketingBar"                                ⁢                                  (                                      Re                    +                    Rfb                                    )                                            )                                                          (        1.2        )            
Using the aforementioned simplified dimensioning rule, according to which fp2xe2x89xa710*fp1 is to apply for a given load, the necessary minimum shunt current and thus the resistance value Rmin (the sum of resistors Re and Rfb) can be calculated.
The second pole fp2 is directly proportional to the output conductance of the driving transistor. The minimum output conductance of the transistor is directly proportional to the minimum shunt current Iq=Is1 set and thus ultimately to the minimum phase margin of the configuration.
As has already been explained above, these relationships are disadvantageous.
For this reason, alternatives for influencing the stability of voltage converters that do not have these disadvantages have long been sought.
One possibility for this consists in providing additional elements by which the transfer function of the system or, more precisely, the position of the pole positions and zero positions of the transfer function can be influenced in order to thus guarantee a minimum phase margin for stabilization purposes. In the case of the voltage regulator shown in FIG. 5, these possibilities have been used. The additional elements contain the resistor Rs and the capacitors Cs1, Cs2 and Cs3. Of the elements, resistor Rs and capacitor Cs1 are connected in series and disposed between the output terminal of the transconductance amplifier OTA1 and ground, the capacitor Cs2 is disposed between the feedback branch and ground, and the capacitor Cs3 is disposed in parallel with the resistor Rfb.
The elements make it possible to influence the position of the pole and zero positions of the transfer function and thus also the stability characteristic of the system. However, it is difficult and complex and in some cases even impossible to dimension the elements in such a manner that the voltage regulator operates in a stable manner over the entire load range.
There are a large number of publications in which these and other possibilities for stabilizing voltage regulators are described. Reference is made, for example, to:
a) Thomas M. Frederiksen: xe2x80x9cA Monolithic High-Power Series Voltage Regulatorxe2x80x9d, IEEE Journal of Solid-State Circuits, December 1968, page 380 ff.;
b) Gabriel A. Rincon-Mora et al.: xe2x80x9cA Low-Voltage, Low Quiescent Current, Low Drop-Out Regulatorxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 33, No. 1, January 1998, pages 36 ff.;
c) Gerrit W. den Besten et al.: xe2x80x9cEmbedded 5 V-to-3.3 V Voltage Regulator for Supplying Digital ICs in 3.3 V CMOS Technologyxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 33, No. 7, July 1998, page 956 ff; and
d) the other references mentioned therein.
Among the known methods for stabilizing voltage regulators, there is none which is simple to configure and implement and can guarantee reliable stabilization with little intrinsic power requirement under all circumstances.
This applies not only to the series voltage regulator described above but also to so-called low drop output (LDO) regulators which have a common-source PMOS transistor as the driving transistor.
It is accordingly an object of the invention to provide a voltage regulator which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which it can guarantee reliable stabilization under all circumstances with minimum intrinsic power requirement and, in addition, is simple to configure and implement.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage regulator. The voltage regulator contains a transistor and an output supplying an output voltage that depends on a drive to the transistor. The output is connected to the transistor. A stabilizing circuit is connected to the transistor for changing a current flowing through the transistor.
The voltage regulator according to the invention is distinguished by the fact that it contains a stabilization circuit that can change the current flowing through the transistor.
The stabilization circuit can ensure that the current flowing through the transistor is increased in phases, specifically only in phases in which it would be too small for guaranteeing stable operation of the voltage regulator.
This dispenses with the necessity of having a high shunt current flowing permanently through the transistor. The voltage regulator can be constructed in such a manner that the shunt current flowing through the transistor is very low in phases in which it is not increased by the stabilization circuit, as a result of which the current flowing through the transistor is only slightly higher with large loads than the current drawn by the load.
This has the positive effect that the transistor can be dimensioned in sole dependence on the maximum load, that is to say it does not have to be made larger for reasons of the stability of the voltage regulator. In addition, the voltage regulator according to the invention has a lower intrinsic power requirement because, of course, the additional shunt current is only caused to flow in particular phases.
Moreover, the stabilization circuit can be simply configured and implemented and can be matched without problems to the respective circumstances. In addition, it can be used essentially unchanged in all types of voltage regulators, the output voltage of which depends on the drive to a transistor.
In accordance with an added feature of the invention, the current flowing through the transistor is changed by changing a load driven by the transistor. The load driven by the transistor is changed by reconfiguring the voltage regulator. The stabilization circuit has a switch coupled to the transistor, and the reconfiguration is effected by opening or closing the switch, and through the switch the transistor can be connected to a component acting as a load element or a current sink.
In accordance with an additional feature of the invention, the stabilization circuit has a component disposed in a circuit branch containing the transistor. The current flowing through the transistor is changed by changing a drive to the component.
In accordance with another feature of the invention, the component is a second transistor connected in series with the transistor. The current flowing through the transistor is changed by changing the drive to the second transistor.
In accordance with a further feature of the invention, the stabilizing circuit has a third transistor interconnected with the second transistor to form a current mirror. A current flowing through the second transistor depends on a current flowing through the third transistor.
In accordance with a further added feature of the invention, the stabilization circuit initiates a change in the current flowing through the transistor when and as long as the current flowing through the transistor has a magnitude at which stable operation of the voltage regulator cannot be guaranteed.
In accordance with a further additional feature of the invention, the stabilizing circuit does not change the current flowing through the transistor when and as long as the current flowing through the transistor has a magnitude at which stable operation of the voltage regulator is guaranteed.
In accordance with another further feature of the invention, the stabilization circuit has a reference current generator outputting a reference current, and the stabilization circuit generates a further current. A magnitude of the further current is a measure of the current flowing through the transistor and changes the current flowing through the transistor when the further current or an additional current depending on the further current is less than the reference current.
In accordance with another added feature of the invention, the stabilization circuit has a fourth transistor driven like the transistor and generates the further current. The fourth transistor is dimensioned to be smaller than the transistor. The stabilization circuit ensures that the fourth transistor is operated at a same operating point as the first transistor. The stabilization circuit has a fifth transistor connected in series with the fourth transistor. The stabilization circuit has a sixth transistor interconnected with the fifth transistor to form a further current mirror. The sixth transistor has a source terminal receiving the reference current, and the source terminal of the sixth transistor is further connected to a primary transistor of the current mirror.
In accordance with a concomitant feature of the invention, the current flowing through the transistor is changed via a hysteresis loop.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage regulator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.